1. Technical Field
This invention relates to the manufacture of ceramic matrix composite parts having cooling channels therein.
2. Background Information
Ceramic matrix composites(xe2x80x9cCMCxe2x80x9d) are well known in the art and may comprise ceramic fibers in a ceramic matrix. CMC are used in high temperature environments, such as in the hot section of a gas turbine engine. For handleability, and to achieve desired mechanical and thermal property orientation in the final product, the ceramic fibers may be woven together to form essentially two-dimensional plies of fiber xe2x80x9cclothxe2x80x9d, or they may be woven into a three dimensional preform of the desired thickness. The preform and matrix are then consolidated in a mold by any of several well know processes, such as by melt infiltration, chemical vapor infiltration, or pre-ceramic polymer processing.
One known CMC composition is made using silicon carbide fibers in a silicon carbide matrix (commonly referred to as a SiC/SiC composite). Another well know CMC composition uses SiC fibers in a silicon-nitrogen-carbon matrix (commonly referred to as a SiC/SiNC composite). Although, using current technology, these and other ceramic matrix composites can withstand temperatures as high as 2200xc2x0 F., they still need to be cooled for today""s gas turbine applications, and improvements are being sought to even further increase their temperature capabilities. Usually the walls of gas turbine engine components are made as thin as possible to minimize weight and to decrease the xe2x80x9cthrough thicknessxe2x80x9d thermal stresses. The wall thickness of hollow CMC turbine vanes is usually no more than about 0.1 inch, and may be considerably less.
A common method for preventing the degradation of components subject to very high temperatures is to flow cooling fluid through channels or passageways within the component. Current CMC fabrication techniques and materials make it difficult to form curved channels and in-plane internal channels having the requisite shapes, locations, tolerances, and dimensions, especially in-plane small diameter channels for thin walled components.
It is well known that straight passages or channels extending from an outer surface of the part to the interior or entirely through the part can be formed by laser machining after consolidating the fiber preform and matrix material; however, laser drilling has its limitations since curved channels cannot be laser drilled, and channels oriented substantially parallel to a wall surface (i.e. in-plane) cannot be made by laser drilling in thin walled parts, unless drilled into an edge. In any event, laser drilling is practical only for straight passages of less than about 0.5 inch long.
Voids or channels of various shapes may also be created within CMC parts by the steps of (a) inserting either carbon rods or graphite paper between the fiber plies as they are being laid-up in the mold, and (b) burning-out (thermal decomposition by oxidation) the carbon after consolidation of the preform and matrix material. The use of carbon rods is not practical for long, narrow or curved channels since they are brittle and mechanically weak (i.e., a 0.02 inch diameter carbon rod has a tensile strength of less then 1.0 ksi), breaking easily during the matrix consolidation step. Graphite paper also has a low mechanical strength and tendency to break during handling and weaving (i.e., a tensile strength of less than about 1.0 ksi). Further, neither the use of carbon rods nor graphite paper is suited to an automated manufacturing process, especially where the channels are long, have small diameters, must be carefully located, may need to interconnect with each other, or may need to have a small radius of curvature. More specifically, in view of manufacturing limitations, it has not been possible to make ceramic matrix composite parts with small diameter (i.e. less than and effective diameter of about 0.10 inch) elongated channels that include a change in direction requiring a radius of curvature smaller than about 6.0 inches. Thus, zig-zag channels could not be formed. Also, it was not possible to form a complex pattern of intersecting elongated channels, such as an intersecting grid of channels within a thin wall. As used herein and in the appended claims, xe2x80x9celongatedxe2x80x9d means having a length to xe2x80x9ceffective diameterxe2x80x9d ratio of at least about 50. Effective diameter is the diameter of a circle having the same cross sectional area as the channel in question.
In the manufacture of a ceramic matrix composite part, inserts having essentially the size and shape of elongated channels to be formed within the part are disposed at a desired location within a woven ceramic fiber preform. The inserts comprise a plurality of carbon fibers surrounded by a carbonaceous filler. After the inserts are in place, a ceramic matrix material is added and the fiber preform is consolidated with the matrix material. The consolidated part is then heated to thermally decompose the inserts leaving elongated channels within the part. The inserts may be rods of carbon fibers or may be flexible, weaveable carbon fiber tows.
During the consolidation step, the carbonaceous filler material fills the interstices between the carbon fibers of the inserts and acts as a protective shell around individual carbon fibers and around any bundles of those fibers that comprise the insert. This inhibits process gases and matrix material from entering voids between the carbon fibers and from directly contacting the carbon fibers. The matrix material, if able to contact the fibers, would inhibit the oxidation of the carbonaceous inserts by depositing a non-oxidizable coating on the fibers, making successful removal of the carbon fiber difficult, if not impossible. Any matrix material that works its way between the carbon fibers will not be removed during thermal decomposition. Thus, by way of example, in the chemical vapor infiltration (CVI) consolidation of a SiC/SiC composite, the carbonaceous filler inhibits the methyltrichlorosilane gas from contacting the carbon fibers and depositing silicon carbide thereon.
As used herein and in the claims, a xe2x80x9ccarbonaceousxe2x80x9d filler material is a material that produces at least about 10% carbon residue, by weight, upon thermal decomposition in a non-oxidizing environment. Preferably the filler will form a high surface area material upon decomposition, such as a film or closed cell foam that will inhibit infiltration of the matrix material between the carbon fibers. Without intending to limit the same, examples of carbonaceous filler materials usable in the process of the present invention are colloidal graphite (with or without a binder) and polymers containing sufficient carbon molecules to satisfy the definition of xe2x80x9ccarbonaceousxe2x80x9d, such as epoxy, silicone, and polyacrylonitrile.
The process of the present invention is particularly suited for (although not limited to) forming in-plane elongated channels of small effective diameter (i.e. small cross sectional area) between the surfaces of thin walled components, and for closely locating and closely spacing those channels. xe2x80x9cSmall effective diameterxe2x80x9d means a cross section having an effective diameter of less than about 0.10 inch. Parts made by the process of the invention are particularly unique in that they may be made with elongated channels of small effective diameter, wherein the channels may have a radius of curvature of less than 1.0 inch, and even less than 0.02 inch. The weavable braided fibers used in the process of the present invention to form the channels have actually been wrapped around a pin of 0.015 inch diameter without major damage and without significant flattening (which would result in a smaller cross sectional channel area around sharp curves). This makes the process of the present invention particularly suited for making thin walled parts having a plurality of curved, elongated channels of small effective diameter. By a xe2x80x9cthin wallxe2x80x9d it is meant a wall having a thickness of 0.25 inch or less.
In accordance with one embodiment of the invention, the inserts used in the fabrication process are individual rods of carbon fibers filled with a thermoset polymeric material that is cured in order that the rods hold a rigid cross-sectional shape and can be handled without breaking. The fibers are preferably continuous, substantially unidirectional carbon fiber tows since continuous unidirectional fibers can be packed more densely into the same rod volume. Rods of this type may be made by pultrusion (xe2x80x9cpultrudedxe2x80x9d rods) and are currently commercially available, although used for applications such as to reinforce fishing poles. Pultruded graphite/epoxy rods (comprising continuous graphite filaments) are commercially available in a variety of diameters from 0.010 inch to 0.04 inch, and higher. They may be made with a variety of cross-sectional shapes, so channels of different cross sectional shapes may be formed. The rods may be inserted by hand into or between plies of woven ceramic fibers; or they may be inserted in the fill direction into either a two or three-dimensional preform during the weaving process. In the latter case, the rods are preferably used where minimal bending is required, such as the fill fibers in an angle interlock weave. That is because rods incorporating a cured thermoset filler have limited flexibility and may not be pliable enough to bend around sharp curvatures without breaking.
In a preferred embodiment, the inserts are flexible, weavable inserts. The weavable inserts are comprised of carbon tows filled with a flexible carbonaceous material rather than a cured thermoset polymeric material. Preferably the tows are braided or twisted to help the insert retain a generally circular or oval cross-sectional shape during the weaving operation. In any case, the filled tows are flexible (as compared to the above described rods made with the cured thermoset polymeric material) and can be woven into a preform by standard weaving machines as either or both warp and weft (fill) filaments.
After consolidating the preform with the matrix material, the carbon tows and carbonaceous filler are removed by thermal decomposition, leaving channels in their place within the CMC part. One particular advantage of being able to essentially xe2x80x9cweavexe2x80x9d a channel in any direction, is that a complex pattern of interconnected channels may be formed; and the channels may be curved and thus may change directions sharply, if desired.
One example of a flexible carbonaceous filler is carbon particles within a carrier, such as a colloid of graphite. Although not required, a binder, such as colloidal silica, is beneficial as it helps keep the carbon particles in place after the fluid carrier has evaporated during processing. Another example of a flexible carbonaceous filler is a thermoplastic polymeric material. RTV silicone is one example of such a material.
In yet another example, the filler may be a partially cured thermoset polymer, which is preferably an epoxy. The filled carbon tows and the ceramic fibers are woven together to form the preform while the thermoset polymer is in a partially cured state prior to full molecular cross-linking. The preform and matrix are then consolidated, during which time the epoxy cures. The channels are then formed by thermally decomposing the filled carbon tows. While the epoxy is in the partially cured state, the carbon tows remain sufficiently flexible to be xe2x80x9cweaveablexe2x80x9d using automated equipment. Further, the use of epoxy avoids residue build-up on the weaving equipment, as may occur when colloidal graphite is the filler. Yet another advantage is that an epoxy does not produce non-oxidizable constituents.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.